The present invention relates generally to electronic image pickup equipment, and more particularly to a video camera or digital camera wherein its thickness in the depthwise direction is reduced by making some contrivance for optical systems such as a zoom lens system. In addition, the zoom lens system is designed to be rear focused.
In recent years, digital cameras (electronic cameras) attract public attention as next-generation cameras now superseding 24 mmxc3x9736 mm film (usually called Leica format) cameras. For current digital cameras there are a wide range of categories from a high-performance type for commercial use to a portable popular type.
A chief object of the present invention is to achieve a video or digital camera of the portable popular type category in particular, which is reduced in depth dimensions while high image quality is ensured.
The greatest bottleneck in reducing the depth dimensions of a camera is the thickness of the surface, nearest to the object side, of an optical system, especially a zoom lens system to an image pickup plane. Recently, a so-called collapsible mount type of lens barrel has gone mainstream, wherein an optical system is driven out of a camera body for phototaking and the optical system is housed in the camera body for carrying. However, the thickness of the lens mount with the optical system housed therein varies largely depending of the lens type used, the filters used or the like. To obtain high specifications especially regarding zoom ratios, F-number, etc., it is preferable to make use of a so-called positive precedent type of zoom lens system wherein the lens group located nearest to its object side has positive refracting power. Even when the zoom lens system is housed in a lens mount, however, it is impossible to reduce the thickness of a camera largely, because the respective lens elements have some thicknesses with a large dead space (see JP-A 11-258507). In this regard, a negative precedent type of zoom lens system, especially a zoom lens system comprising two or three lens groups is favorable. However, it is still impossible to reduce the thickness of a camera largely, even when the lens nearest to the object side is a positive lens. This is because each lens group comprises a number of lens elements or the lens elements are thick (see JP-A 11-52246). Some known examples of the zoom lens system suitable for use with electronic image pickup devices, having satisfcatory image-formation capabilities inclusive of zoom ratios, field angles and F-numbers and capable of having the smallest thickness of a lens mount with the zoom lens system housed therein are disclosed in JP-A""s 11-194274, 11-287953 and 2000-9997.
To make the first lens group thin, it is preferable to locate an entrance pupil at a shallow position. To this end, on the one hand, it is required to increase the magnification of the second lens group. On the other hand, some considerable burdens are placed on the second lens group. This does not only make it difficult to keep the second lens group thin but also to make correction for aberrations, resulting in an unacceptably increase in the influence of fabrication errors. Thickness and size reductions may be achieved by reducing image pickup device size. To achieve the same number of pixels, however, it is required to reduce pixel size and make up for sensitivity shortages by the optical system. The same also holds for the influence of diffraction.
To reduce the depth dimensions of a camera body, it is preferable in view of a driving mechanism layout to make use of a rear focusing mode wherein the movement of lenses for focusing is carried out by a rear lens group rather than a front lens group. In this case, however, it is required to make a selection from optical systems less susceptible to aberration fluctuations in the rear focusing mode.
In view of such problems with the prior art as explained above, it is a primary object of the present invention to reduce the thickness of electronic image pickup equipment as much as possible by making selective use of a zoom mode or construction having a compact yet simple mechanism layout and stable yet high image-formation capabilities from an object at infinity to a near-by object, for instance, a rear focusing mode having a reduced number of lens elements, and making lens elements so thin that the total thickness of each lens group can be reduced while the selection of filters is taken into account.
According to the first aspect of the present invention, this object is achieved by the provision of electronic image pickup equipment including a zoom lens system and an electronic image pickup device in the rear of said zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, in which for zooming from a wide-angle end to a telephoto end of the zoom lens system upon focused on an object point at infinity, a separation between the second lens group and the third lens group becomes wide and which can be focused at a nearer-by subject by moving the third lens group toward the object side, characterized in that:
said second lens group comprises, in order from an object side thereof, one positive lens 2a, one negative lens 2b and a lens subgroup 2c comprising at least one lens and said third lens group comprises one positive lens, while the following conditions are satisfied:
0.04 less than t2N/t2 less than 0.18xe2x80x83xe2x80x83(1)
xe2x88x920.5 less than f2a/f2c less than 1.1xe2x80x83xe2x80x83(2)
where t2N is an axial distance from an image-side surface of the positive lens 2a located on the object side of the second lens group to an image-side surface of the negative lens 2b in the second lens group, t2 is an optical axis distance from an object-side surface of the positive lens 2a located on the object side of the second lens group to a surface located nearest to an image side of the lens subgroup 2c, and f2a, and f2c is a focal length in air of the positive lens 2a located on the object side of the second lens group, and the lens subgroup 2c, respectively.
According to the second aspect of the present invention, there is provided electronic image pickup equipment including a zoom lens system and an electronic image pickup device in the rear of said zoom lens comprising, in order from an object side of the zoom lens system, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, in which for zooming from a wide angle end to a telephoto end of the zoom lens system upon focused on an object point at infinity, a separation between the second lens group and the third lens group becomes wide and which can be focused at a nearer-by subject by moving the third lens group toward the object side, characterized in that:
said second lens group comprises, in order from an object side thereof, one positive lens 2a, one negative lens 2b and a lens group 2c consisting of one lens and said third lens group comprises one positive lens, while the following conditions are satisfied:
0.04 less than t2N/t2 less than 0.18xe2x80x83xe2x80x83(1)
xe2x80x83xe2x88x920.5 less than f2a/f2c less than 1.1xe2x80x83xe2x80x83(2)
where t2N is an optical axis distance from an image-side surface of the positive lens 2a located on the object side of the second lens group to an image-side surface of the negative lens 2b in the second lens group, t2 is an optical axis distance from an object-side surface of the positive lens 2a located on the object side of the second lens group to a surface located nearest to an image side of the lens group 2c, and f2a, and f2c is a focal length in air of the positive lens 2a located on the object side of the second lens group, and the lens subgroup 2c, respectively.
An account is now given of why the aforesaid arrangements are used in the present invention and how they work.
The electronic image pickup equipment of the present invention includes a zoom lens system comprising, in order from the object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power. For zooming from the wide-angle end to the telephoto end of the system upon focused on an object point at infinity, the separation between the second lens group and the third lens group becomes wide. By moving the third lens group toward the object side of the system, the system can be focused on a nearer-by subject. The second lens group comprises, in order from the object side thereof, one positive lens 2a, one negative lens 2c and a lens subgroup 2c comprising at least one lens including an aspherical surface, and the third lens group comprises one positive lens.
Alternatively, the second lens group may comprise, in order from the object side thereof, one positive lens 2a, one negative lens 2b and a lens subgroup 2c consisting of one lens including an aspherical surface, and the third lens group may comprise one positive lens.
This requirement for the zoom lens system according to the present invention is inevitable for reducing fluctuations of off-axis aberrations including astigmatism with focusing by the third lens group while the total thickness of the lens portion during lens housing is kept thin.
For an electronic image pickup device, it is required to reduce the angle of incident rays as much as possible. A positive lens in a two-group zoom lens system of + xe2x88x92 construction most commonly used as a silver salt camera-oriented zoom lens system, which positive lens is located nearest to the image side thereof, is used as a third lens group designed to be independently movable in such a way as to keep an exit pupil at a farther position. When this third lens group is used for focusing purposes, aberration fluctuations offer a problem. When asphericity is incorporated in the third lens group in an amount larger than necessary, it is required that astigmatism remaining at the first and second lens groups be corrected by the third lens group so as to obtain an aspheric effect. In this case, it is not preferable to move the third lens group for focusing, because the correction of astigmatism becomes out of balance. In order to carry out focusing with the third lens group, it is therefore required to substantially remove the astigmatism at the first and second lens groups over all the zooming zone. For this reason, it is desired that the third lens group be made up of a spherical element or an element having a small amount of asphericity, an aperture stop be located on the object side of the second lens group, and an aspherical surface be used at a lens in the second lens group, which lens is positioned nearest to the image side of the second lens group and has a particular effect on off-axis aberrations. In addition, since this type zoom lens system makes it difficult to increase the diameter of the front lens, it is preferable to make an aperture stop integral with the second lens group (as can be seen from the examples, given later, wherein the aperture stop is located just before the second lens group for integration therewith). This arrangement is not only simple in mechanism but also makes any dead space less likely to occur during lens housing, with a reduced F-number difference between the wide-angle end and the telephoto end.
In the present invention, the following conditions (1) and (2) should be satisfied.
0.04 less than t2N/t2 less than 0.18xe2x80x83xe2x80x83(1)
xe2x88x920.5 less than f2a/f2c less than 1.1xe2x80x83xe2x80x83(2)
where t2N is the optical axis distance from the image-side surface of the positive lens 2a located on the object side of the second lens group to the image-side surface of the negative lens 2b in the second lens group, t2 is the optical axis distance from the object-side surface of the positive lens 2a located on the object side of the second lens group to the surface located nearest to the image side of the lens subgroup 2c, and f2a, and f2c is the focal length in air of the positive lens 2a located on the object side of the second lens group, and the lens subgroup 2c, respectively.
Condition (1) gives a definition of t2N that is the optical axis distance from the image-side surface of the positive lens 2a located on the object side of the second lens group to the image-side surface of the negative lens 2b in the second lens group. Unless this site has a certain thickness, astigmatism cannot perfectly be corrected. However, this thickness becomes an obstacle to making each element of the optical system thin. Thus, the astigmatism is corrected by introducing an aspherical surface in the image-sides surface of the lens located on the image side. Nonetheless, when the lower limit of 0.04 is not reached, the astigmatism remains undercorrected. When the upper limit of 0.18 is exceeded, the thickness becomes unacceptably large.
Condition (2) gives a definition of the focal length ratio in air between the positive lens 2a on the object side of the second lens group and the lens subgroup 2c. When the upper limit of 1.1 is exceeded, the principal points of the second lens group are shifted to the image side; some dead space is likely to occur in the rear of the second lens group when the system is in use, resulting in an increase in the overall length of the system. To make the system thin upon lens housing in this case, it is thus necessary to use a more complicated or larger lens barrel mechanism. Otherwise, it is impossible to make the thickness of the lens barrel mechanism thin to a certain degree. When the lower limit of xe2x88x920.5 is not reached, correction of astigmatism becomes difficult.
More preferably, conditions (1) and (2) should be:
0.05 less than t2N/t2 less than 0.16xe2x80x83xe2x80x83(1)xe2x80x2
xe2x88x920.4 less than f2a/f2c less than 0.8xe2x80x83xe2x80x83(2)xe2x80x2
Most preferably, conditions (1) and (2) should be:
0.06 less than t2N/t2 less than 0.15xe2x80x83xe2x80x83(1)xe2x80x3
xe2x88x920.3 less than f2a/f2c less than 0.62xe2x80x83xe2x80x83(2)xe2x80x3
As already mentioned, it s desired that the lens subgroup 2c in the second lens group comprise an aspherical surface and the third lens group consist only of a spherical surface or an aspherical surface that satisfies the following condition:
abs(z)/L less than 1.5xc3x9710xe2x88x922xe2x80x83xe2x80x83(3)
Here abs(z) is the absolute value of the amount of a deviation of the aspherical surface in the third lens group from a spherical surface having an axial radius of curvature in the optical axis direction as measured at a height of 0.35 L from the optical axis, and L is the diagonal length of an effective image pickup plane.
Exceeding the upper limit of 1.5xc3x9710xe2x88x922 to condition (3) is not preferable, because astigmatism is largely out of balance upon rear focusing with the third lens group.
More preferably, condition (3) should be:
abs(z)/L less than 1.5xc3x9710xe2x88x923xe2x80x83xe2x80x83(3)xe2x80x2
Most preferably, condition (3) should be:
abs(z)/L less than 1.5xc3x9710xe2x88x924xe2x80x83xe2x80x83(3)xe2x80x3
In addition, it is preferable to satisfy the following conditions (4) and (5). This is because even when rear focusing is introduced in the optical system while it is kept thin, various aberrations such as astigmatism and chromatic aberrations remain stable all over the zooming zone from an object at infinity to a near-by object.
(R2cl+R2cr)/(R2clxe2x88x92R2cr) less than xe2x88x920.4xe2x80x83xe2x80x83(4)
xe2x88x921.1 less than (R31+R32)/(R31xe2x88x92R32) less than 1.5xe2x80x83xe2x80x83(5)
Here R2cl and R2cr are the axial radii of curvature of the surfaces in the image-side lens subgroup 2c in the second lens group, which surfaces are located nearest to the object and image sides, respectively, and R31 and R32 are the axial radii of curvatures of the first and second lens surfaces in the third lens group, respectively, as counted from the object side.
Conditions (4) and (5) give definitions of the shape factors of the aspherical lens subgroup 2c of the second lens group, which is located nearest to the image side thereof and the positive lens in the third lens group. When the upper limit of 1.5 to condition (5) is exceeded, fluctuations of astigmatism due to rear focusing become too large, and the astigmatism is likely to becoming worse with respect a near-by object point although the astigmatism may be well corrected on an object point at infinity. When the upper limit of xe2x88x920.4 to condition (4) is exceeded and the lower limit of xe2x88x921.1 to condition (5) is not reached, the fluctuations of astigmatism due to rear focusing are reduced; however, it is difficult to make correction for aberrations on an object point at infinity.
More preferably, conditions (4) and (5) should be:
xe2x88x9210.0 less than (R2cl+R2cr)/(R2clxe2x88x92R2cr) less than xe2x88x920.6xe2x80x83xe2x80x83(4)xe2x80x2
xe2x88x920.5 less than (R31+R32)/(R31xe2x88x92R32) less than 1.2xe2x80x83xe2x80x83(5)xe2x80x2
When the lower limit of xe2x88x9210.0 to condition (4)xe2x80x2 is not reached, the fluctuations of astigmatism due to rear focusing become large.
Most preferably, conditions (4) and (5) should be:
xe2x88x925.0 less than (R2cl+R2cr)/(R2clxe2x88x92R2cr) less than xe2x88x920.8xe2x80x83xe2x80x83(4)xe2x80x3
0.1 less than (R31+R32)/(R31xe2x88x92R32) less than 1.0xe2x80x83xe2x80x83(5)
In the second lens group, the positive and negative lenses located on its object side should preferably be cemented together, because some considerable aberrations occur due to their relative decentration. In addition, the second lens group comprises one negative lens adjacent to both positive lenses, wherein the negative lens is cemented to either one of the positive lenses. In this case, the third lens group may comprise one positive lens composed of only spherical surfaces.
It is here noted that when the lens subgroup 2c of the second lens group comprises a single lens, the cemented lens consisting of lenses 2a and 2b should preferably satisfy the following condition (6):
xe2x88x921.5 less than {(R2a1+R2a2)xc2x7(R2b1xe2x88x92R2b2)}/{(R2a1xe2x88x92R2a2)xc2x7(R2b1+R2b2)} less than xe2x88x920.6xe2x80x83xe2x80x83(6)
Here R2a1 and R2a2 are the axial radii of curvature on the object and image sides, respectively, of the lens 2a in the second lens group, and R2b1, and R2b2 are the axial radii of curvature on the object and image sides, respectively, of the lens 2b in the second lens group.
Condition (6) gives a definition of the shape factor ratio between the lens elements (positive lens and negative lens) of the cemented lens in the second lens group. Falling below the lower limit of xe2x88x921.5 to condition (6) is unfavorable for correction of longitudinal chromatic aberration and exceeding the upper limit of xe2x88x920.6 is unfavorable for size reductions because the lens elements become thick.
More preferably, condition (6) should be:
xe2x88x921.3 less than {(R2a1+R2a2)xc2x7R2b1xe2x88x92R2b2)}/{(R2a1xe2x88x92R2a2)xc2x7(R2b1+R2b2)} less than xe2x88x920.7 xe2x80x83xe2x80x83(6)xe2x80x2
Most preferably, condition (6) should be:
xe2x88x921.2 less than {(R2a1+R2a2)xc2x7(R2b1xe2x88x92R2b2)}/{(R2a1xe2x88x92R2a2)xc2x7(R2b1+R2b2)} less than xe2x88x920.8xe2x80x83xe2x80x83(6)xe2x80x3
A zoom lens system having a zoom ratio of 2.3 or greater, if it satisfies the following conditions, can then make some contribution to thickness reductions.
1.3  less than xe2x88x92xcex22t less than 2.1xe2x80x83xe2x80x83(a)
1.6 less than f2/fW less than 3.0xe2x80x83xe2x80x83(b)
Here xcex22t is the magnification of the second lens group at the telephoto end (an object point at infinity), f2 is the focal length of the second lens group, and fW is the focal length of the zoom lens system at the wide-angle end (an object point at infinity).
Condition (a) gives a definition of the magnification xcex22t of the second lens group at the telephoto end (when the zoom lens system is focused on an object point at infinity). The larger this absolute value, the easier it is to reduce the diameter of the first lens group because it is possible to make shallow the position of the entrance pupil at the wide-angle end, and so the smaller the first lens group is. When the lower limit of 1.3 is not reached, it is difficult to satisfy thickness. When the upper limit of 2.1 is exceeded, it is difficult to make correction for various aberrations (spherical aberrations, coma and astigmatism). Condition (b) gives a definition of the focal length f2 of the second lens group. To reduce the thickness of the second lens group itself, the focal length of the second lens group should preferably be reduced as much as possible. In view of power profile, however, this is unreasonable for correction of the aberrations because the front principal point of the second lens group is positioned on the object side while the rear principal point of the first lens group is positioned on the image side. When the lower limit of 1.6 is not reached, it is difficult to make correction for spherical aberrations, coma, astigmatism, etc. When the upper limit of 3.0 is exceeded, it is difficult to achieve thickness reductions.
More preferably, conditions (a) and (b) should be:
1.4  less than xe2x88x92xcex22t less than 2.0xe2x80x83xe2x80x83(a)xe2x80x2
1.8 less than f2/fW less than 2.7xe2x80x83xe2x80x83(b)xe2x80x2
Most preferably, conditions (a) and (b) should be:
1.5  less than xe2x88x92xcex22t less than 1.9xe2x80x83xe2x80x83(a)xe2x80x3
xe2x80x832.0 less than f2/fW less than 2.5xe2x80x83xe2x80x83(b)xe2x80x3
Thus, thickness reductions are contradictory to correction of aberrations, and so it is preferable to introduce an aspherical surface in the positive lens in the second lens group, which positive lens is positioned nearest to its object side. This aspherical surface has a great effect on correction of spherical aberrations and coma, so that astigmatism and longitudinal chromatic aberration can favorably be corrected. Preferably in this case, condition (6) or (6)xe2x80x2 or (6)xe2x80x3 should be satisfied as well irrespective of the construction of the second lens group.
As already explained, when rear focusing is carried out with the third lens group, correction of off-axis aberrations should preferably be substantially completed with the first and second lens groups all over the zooming zone. If the construction of the first lens group is selected with the construction of the second lens group in mind, it is then possible to substantially complete the correction of off-axis aberrations with the first and second lens groups all over the zooming zone. The then construction of the first lens group is now explained.
The first embodiment of the first lens group comprises, in order from the object side thereof, a negative lens subgroup comprising up to two negative lenses and a positive lens subgroup consisting of one positive lens. In the first embodiment, at least one negative lens in the negative lens subgroup comprises an aspherical surface and the following conditions (7) and (8) are satisfied.
The second embodiment comprises, in order from the object side thereof, one positive lens, two negative lenses and one positive lens and optionally satisfies the following condition (9).
The third embodiment of the first lens group comprises, in order from the object side thereof, one positive lens, one negative lens and one positive lens. In the third embodiment, either one of the positive lenses comprises an aspherical surface and has a weak refracting power and the following condition (10) is satisfied.
The fourth embodiment comprises, in order from the object side thereof, two negative lenses, one positive lens and one negative lens.
In the present invention, any one of the aforesaid four embodiments should preferably be used for the first lens group. The aforesaid conditions (7) through (10) are now explained.
xe2x88x920.1 less than fW/R11 less than 0.45xe2x80x83xe2x80x83(7)
0.13 less than dNP/fW less than 1.0xe2x80x83xe2x80x83(8)
0.75 less than R14/L less than 3xe2x80x83xe2x80x83(9)
0 less than fW/f1P less than 0.3xe2x80x83xe2x80x83(10)
Here R11 is the axial radius of curvature of the first lens surface in the first lens group, as counted from the object side, fW is the focal length of the zoom lens system at the wide-angle end (when focused on an object point at infinity), dNP is the axial air separation between the negative and positive lens subgroups of the first lens group, R14 is the axial radius of curvature of the fourth lens surface in the first lens group, as counted from the object side, L is the diagonal length of the effective image pickup area of the image pickup device, f1P is the focal length of the positive lens in the first lens group, which lens comprises an aspherical surface and has a weak refracting power, and fW is the focal length of the zoom lens system at the wide-angle end (when focused on an object point at infinity).
Condition (7) gives a definition of the radius of curvature of the first surface in the first embodiment of the first lens group. It is preferable that distortion is corrected by introducing the aspherical surface in the first lens group and astigmatism is corrected by the remaining spherical component. Exceeding the upper limit of 0.45 is unfavorable for correction of the astigmatism, and when the lower limit of xe2x88x920.1 is not reached, the distortion cannot perfectly be corrected even by the aspherical surface.
Condition (8) gives a definition of the axial air separation dNP between the negative lens subgroup and the positive lens subgroup in the first embodiment of the first lens group. Exceeding the upper limit of 1.0 may be favorable for correction of astigmatism; however, this is contradictory to size reductions because of an increase in the thickness of the first lens group. When the lower limit of 0.13 is not reached, it is difficult to make correction for astigmatism.
Condition (9) gives a definition of the axial radius of curvature R14 of the fourth lens surface in the second embodiment of the first lens group. This embodiment may be favorable for satisfactory correction of astigmatism and distortion; however, the first lens group tends to become thick. If R14 is as large as possible, it is then possible to reduce the thickness of the first lens group. Falling below the lower limit of 0.75 is not preferable because some excessive space is needed. When the upper limit of 3 is exceeded, the first lens group rather increases in diameter and thickness because it is lacking in power.
Condition (10) gives a definition of the focal length f1P of the positive lens in the third embodiment of the first lens group, which lens comprises an aspherical surface and has a weak refracting power. When the upper limit of 0.3 is exceeded, the power of only one negative lens in the first lens group becomes too strong to correct distortion and the concave surface becomes hard-to-process because its radius of curvature becomes too small. Falling below the lower limit of 0 is not preferable in view of correction of astigmatism, because the aspherical surface contributes to only correction of distortion.
More preferably, conditions (7), (8), (9) and (10) should be:
xe2x88x920.05 less than fW/R11 less than 0.25xe2x80x83xe2x80x83(7)xe2x80x2
0.3 less than dNP/fW less than 0.9xe2x80x83xe2x80x83(8)xe2x80x2
0.98 less than R14/L less than 2.5xe2x80x83xe2x80x83(9)xe2x80x2
0 less than fW/f1P less than 0.2xe2x80x83xe2x80x83(10)xe2x80x2
Most preferably, conditions (7), (8), (9) and (10) should be:
xe2x88x920.03 less than fW/R11 less than 0.15xe2x80x83xe2x80x83(7)xe2x80x3
xe2x80x830.32 less than dNP/fW less than 0.8xe2x80x83xe2x80x83(8)xe2x80x3
1 less than R14/L less than 2xe2x80x83xe2x80x83(9)xe2x80x3
0 less than fW/f1P less than 0.1xe2x80x83xe2x80x83(10)xe2x80x3
In the aforesaid second embodiment, the first lens group may comprise, in order from its object side, one positive lens, one negative meniscus lens and a cemented lens component consisting of a negative lens and a positive lens. When the first lens group is made up of four lenses, for instance, a positive lens, a negative lens, a negative lens and a positive lens in this order or two negative lenses, a positive lens and a negative lens in this order, the relative decentration of the two lenses located on the image side often incurs a deterioration in image-formation capabilities. For improvements in centering capabilities, it is thus preferable to cement these lenses together.
In addition, the total thickness of the first lens group, and the second lens group should preferably satisfy the following conditions.
0.4 less than t1/L less than 2.2xe2x80x83xe2x80x83(11)
0.5 less than t2/L less than 1.5xe2x80x83xe2x80x83(12)
Here t1 is the axial thickness of the first lens group from the lens surface located nearest to its object side to the lens surface located nearest to its image side, t2 is the axial thickness of the second lens group from the lens surface located nearest to its object side to the lens surface located nearest to its image side, and L is the diagonal length of the effective image pickup area of the image pickup device.
Conditions (11) and (12) give a definition of the total thickness of the first lens group, and the second lens group, respectively. Exceeding the respective upper limits of 2.2 and 1.5 is likely to form an impediment to size reductions. When the respective lower limits of 0.4 and 0.5 are not reached, it is difficult to set up appropriate paraxial relations or make correction for various aberrations because it is required to moderate the radius of curvature of each lens surface.
In view of marginal thickness and mechanism space, it is here noted that the ranges of these conditions should preferably be adjusted depending on the value of L.
To be more specific, it is desired to satisfy the following conditions (11)xe2x80x2 and (12)xe2x80x2.
Condition (11)xe2x80x2:
When Lxe2x89xa66.2 mm, 0.8 less than t1/L less than 2.2
When 6.2 mm less than Lxe2x89xa69.2 mm, 0.7 less than t1/L less than 2.0
When 9.2 mm less than L, 0.6 less than t1/L less than 1.8
Condition (12)xe2x80x2:
When Lxe2x89xa66.2 mm, 0.5 less than t2/L less than 1.5
When 6.2 mm less than Lxe2x89xa69.2 mm, 0.4 less than t2/L less than 1.3
When 9.2 mm less than L, 0.3 less than t2/L less than 1.1
According to the present invention, it is thus possible to provide means for improving the image-formation capabilities of the zoom lens system while the thickness of the lens mount is reduced.
An account is now given of the conditions for making filters, etc. thin. In electronic image pickup equipment, usually, an infrared absorption filter having such a certain thickness as to prevent incidence of infrared light on an image pickup plane is inserted between an image pickup device and the object side of the equipment. Here consider the case where this filter is replaced by a coating that is substantially devoid of thickness. As a matter of course, the equipment becomes thin by this amount, and there is a spillover effect. When a near-infrared sharp cut coating having a transmittance of at least 80% at 600 nm wavelength and at most 10% at 700 nm wavelength is introduced between the image pickup device in the rear of a zoom lens system and the object side of the equipment, red transmittance is relatively higher than that of an adsorption type, so that the tendency of bluish purple to change to magentaxe2x80x94which is one defect of a CCD having a complementary color mosaic filterxe2x80x94can be mitigated by gain control, thereby achieving color reproduction comparable to that by a CCD having a primary color filter.
On the other hand, a CCD with a complementary color filter mounted thereon, because of its high transmitted light energy, is higher in substantial sensitivity, and more favorable in resolution, than a CCD with a primary color filter mounted thereon. Thus, there is much merit in using the complementary color filter on a CCD of miniature size. Another filter or an optical low-pass filter, too, should preferably satisfy the following condition with respect to its total thickness tLPF.
0.15xc3x97103 less than tLPF/a less than 0.45xc3x97103xe2x80x83xe2x80x83(13)
Here a is the horizontal pixel pitch of an electronic image pickup device.
To make an optical low-pass filter thin, too, is effective for reducing the thickness of the lens mount. However, this is generally not preferable because the effect of the low-pass filter on moire reductions becomes slender. As the pixel pitch becomes small, on the other hand, the contrast of frequency components exceeding Nyquist threshold decreases under the influence of diffraction by an image-formation lens system, so that the decrease in the moire-reducing effect can be accepted to some degrees. For instance, when use is made of three types of filter elements put one upon another in the optical axis direction, each of which elements has crystallographic axes in the azimuth directions of horizontal (=0xc2x0) and xc2x145xc2x0 upon projection on an image plane, it is known that some effects on moire reductions are achievable. Referring here to the specifications where the filter becomes thinnest, it is known that the elements are shifted by axcexcm in the horizontal direction and by SQRT(xc2xd)xc3x97a xcexcm in the xc2x145xc2x0 direction. The then filter thickness amounts to about [1+2xc3x97SQRT(xc2xd)]xc3x97a/5.88 (mm) where SQRT means a square root. This is just the specification where contrast is reduced down to zero at a frequency corresponding to Nyquist threshold.
When the film thickness is smaller than this by a few % to several tens %, there is a contrast of the frequency corresponding to Nyquist threshold. However, this contrast can be controlled by the aforesaid influence of diffraction. Regarding other filter specifications, for instance, when one or two filter elements are used, too, it is preferable to conform to condition (13). When the upper limit of 0.45xc3x97103 is exceeded, the optical low-pass filter becomes too thick to achieve thickness reductions. When the lower limit of 0.15xc3x97103 is not reached, moire removal becomes insufficient. Still, it is required that a be 5 xcexcm or less.
When a is 4 xcexcm or less, it is preferable that
0.13xc3x97103 less than tLPF/a less than 0.42xc3x97103xe2x80x83xe2x80x83(13)xe2x80x2
This is because the optical low-pass filter is more susceptible to diffraction. The optical low-pass filter may then be embodied as follows.
When the low-pass filter is made up of three low-pass filter elements put one upon another and 4 xcexcmxe2x89xa6a less than 5 xcexcm, it is preferable that
0.3xc3x97103 less than tLPF/a less than 0.4xc3x97103xe2x80x83xe2x80x83(13-1)
When the low-pass filter is made up of two low-pass filter elements put one upon another and 4 xcexcmxe2x89xa6a less than 5 xcexcm, it is preferable that
0.2xc3x97103 less than tLPF/a less than 0.28xc3x97103xe2x80x83xe2x80x83(13-2)
When the low-pass filter is made up of one low-pass filter element and 4 xcexcmxe2x89xa6a less than 5 xcexcm, it is preferable that
0.1xc3x97103 less than tLPF/a less than 0.16xc3x97103xe2x80x83xe2x80x83(13-3)
When the low-pass filter is made up of three low-pass filter elements put one upon another and a  less than 4 xcexcm, it is preferable that
0.25xc3x97103 less than tLPF/a less than 0.37xc3x97103xe2x80x83xe2x80x83(13-4)
When the low-pass filter is made up of two low-pass filter elements put one upon another and a  less than 3 xcexcm, it is preferable that
0.6xc3x97103 less than tLPF/a less than 0.25xc3x97103xe2x80x83xe2x80x83(13-5)
When the low-pass filter is made up of one low-pass filter element and a  less than 4 xcexcm, it is preferable that
0.08xc3x97103 less than tLPF/a less than 0.14xc3x97103xe2x80x83xe2x80x83(13-6)
When an image pickup device having a small pixel pitch is used, image quality deteriorates under the influence of diffraction due to stop-down. To avoid this, the present invention provides electronic image pickup equipment, wherein aperture size comprises a plurality of fixed apertures, one out of which can be inserted in an optical path between a lens surface in the first lens group, which surface is nearest to an image side thereof, and a lens surface in the third lens group, which surface is nearest to an object side thereof, and can be replaced with another aperture, so that field illuminance can be controlled. Preferably in this electronic image pickup equipment, some of said plurality of apertures should contain therein media having a varying transmittance of less than 80% with respect to 550 nm wavelength, so that light quantity control can be achieved, and some should contain therein media having a transmittance of 80% or greater with respect to 550 nm.
Alternatively, when control is carried out to obtain a light quantity corresponding to such an F-number as to provide a/F-number  less than 0.4 xcexcm, the apertures should preferably contain therein media having a varying transmittance of less than 80% with respect to 550 nm wavelength.
To put it another way, when control is carried out to obtain a light quantity corresponding to such an effective F-number as to provide FNOxe2x80x2 greater than a/0.4 xcexcm where FNOxe2x80x2 is an effective F-number defined by FNO/T wherein FNO is an F-number found from the focal length of the zoom lens system and the diameter of an entrance pupil and T is an aperture transmittance at 550 nm and a is a horizontal pixel pitch of an electronic image pickup device, it is preferable to insert an aperture containing therein a medium having a transmittance T of less than 80% with respect to 550 nm in a zoom lens optical path.
For instance, when there is a deviation from the aforesaid range on the basis of the open aperture value, the medium may be not used or a dummy medium having a transmittance of 91% or greater with respect to 550 nm wavelength is used. In the aforesaid range, light quantity control may be carried out by using a member such an ND filter rather than decreasing the diameter of the aperture stop to such a degree that the influence of diffraction manifests itself.
Alternatively, optical low-pass filters with varying frequency characteristics instead of ND filters may be inserted in a plurality of apertures whose diameters are evenly reduced in inversely proportional to the F-number. Since the deterioration due to diffraction becomes large with stop-down, it is required that the smaller the aperture diameter, the higher the frequency characteristics of the optical filters be. The higher frequency characteristics mean that the contrast of the spatial frequency of the object image is kept higher than those of other spatial frequencies. In other words, this means that the cutoff frequency is high.
It is here noted that the zoom lens system of the present invention can have a zoom ratio of 2.3 or greater. According to the invention, it is further possible to achieve electronic image pickup equipment comprising a zoom lens system having a zoom ratio of 2.6 or greater.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.